The invention relates to a semi-aromatic polyamide that contains tetramethylene terephthalamide units. The invention also relates to a process for preparing a polyamide that contains at least tetramethylene terephthalamide units and to compositions and products containing said polyamide.
In the context of this application a xe2x80x98semi-aromatic polyamidexe2x80x99 is understood to be a polyamide homo- or copolymer that contains aromatic or semi-aromatic units derived from an aromatic dicarboxylic acid, an aromatic diamine or an aromatic aminocarboxylic acid, the content of said units being at least 50 mol %.
Polytetramethylene terephthalamide (or polyamide 4T, also referred to as nylon 4T, hereinafter abbreviated as PA 4T) is known from for example R. J. Gaymans et al., J.Polym.Sci., Polym.Chem.Ed., 22, 1373 (1984). PA 4T is a semi-aromatic, semi-crystalline polyamide with a melting point higher than about 430xc2x0 C.
Semi-crystalline (co)polyamides are used in particular for moulded parts that are exposed to high temperatures for some time and which parts should then show high dimensional stability and high retention of mechanical properties. On account of increasingly stringent requirements in for example the automotive and electronics industries, there is a constant need for materials that are resistant to high temperatures, preferably a resistance to above approximately 290xc2x0 C., and more preferably to above 300xc2x0 C. Polymers that are suitable for such applications will need to have a melting point that is distinctly higher than approximately 290xc2x0 C. and also a melt stability and crystallisation behaviour such that melt processing via known processes to form moulded parts with a high crystallinity and good mechanical properties is possible.
In the context of this application, xe2x80x98high crystallinityxe2x80x99 is understood to mean a minimum melting enthalpy value of 50 J/g (as derived from a 2nd heating curve obtained with differential scanning calorimetry (DSC) at a heating rate of 20xc2x0 C./min). A high crystallinity presents advantages with respect to, for example, fast processing from the melt into moulded parts and with respect to properties like stiffness and dimensional stability, in particular at temperatures up to a little under the melting point.
A disadvantage of the semi-aromatic polyamide 4T is that it does not show the aforementioned combination of properties. Polyamide 4T has a melting point that is higher than its decomposition temperature, so PA 4T is not a melt-processable polymer, and is, therefore, not suitable for the production of moulded parts via for example an injection-moulding process.
The object of the present invention is to provide a melt-processable semi-aromatic polyamide based on PA 4T that does not show the aforementioned disadvantages, or shows them to a lesser extent.
This object is achieved with a semi-aromatic copolyamide that, in addition to tetramethylene terephthalamidetetramethylene terephthalamide units, also contains hexamethylene terephthalamide units.
It is known per se that the melting temperature of a semi-crystalline, semi-aromatic polyamide can be changed by turning it into a copolymer; see for example Chapter 6 in Polyamide, Kunststoff Handbuch 3/4, Becker/Braun (Eds), Hanser Verlag (Munchen), 1998, ISBN 3-446-16486-3. This textbook teaches that a monomer unit of a different polymer with a lower melting point is usually chosen to lower the melting point of a semi-crystalline polymer, and that a decrease in the melting point is generally accompanied by a substantial, or even complete, loss of crystallinity, in particular at higher comonomer contents (in the order of 30-70 mol %), unless an isomorphic monomer unit is used as the comonomer.
Most surprisingly it has been found that the copolyamide according to the invention nevertheless shows a high crystallinity, in spite of its relatively high non-isomorphic comonomer content. Also surprising is that the copolyamide according to the invention is melt-processable, whereas neither PA 4T nor PA 6T are melt-processable, because the melting points of these two polyamides, which are higher than approximately 430xc2x0 C. and approximately 370xc2x0 C., respectively, lie above their decomposition temperatures. Another advantage is high thermal stability of the copolyamide. Yet another advantage is low water absorption of the copolyamide according to the invention. The monomers of the polyamide according to the invention are moreover commercially available on an industrial scale at low costs.
The copolyamide according to invention preferably contains approximately 30-75 mol % hexamethylene terephthalamide units. The advantage of this is that it can then well be melt-processed without the mechanical properties decreasing too much at a high temperature. In the case of a copolyamide with a concentration of hexamethylene terephthalamide units that is lower or higher than the aforementioned values, the melting point will be too close to the melting points of PA 4T or PA 6T, respectively.
The copolyamide according to the invention optionally also contains units derived from at least one dicarboxylic acid other than terephthalic dicarboxylic acid and/or derived from at least one diamine other than tetramethylene diamine or hexamethylene diamine and/or derived from at least one aminocarboxylic acid or cyclic amide. Also monomers with higher functionalities may optionally also be added. Preferably the copolyamide contains approximately 0.01-20 mol % of the aforementioned units. An advantage of using such additional units is that the melting point can be lowered even further, so that an optimum compromise can be obtained between melt processability on the one hand and retention of crystallinity and mechanical properties on the other. Another advantage is achieved in that milder conditions can be chosen for preparation of the copolyamide. An additional advantage of using monomers, for example acids or amines, with higher functionalities, is that a polymer of higher viscosity can be obtained more readily. To prevent substantial crosslinking the content of such monomers is preferably 0.01-3 mol %. The nature and amounts of additional units can easily be determined by a person skilled in the art on the basis of experiments.
Suitable dicarboxylic acids are for example aromatic dicarboxylic acids or aliphatic dicarboxylic acids with 6 to 18 carbon atoms. Preferably the dicarboxylic acid is an aromatic dicarboxylic acid, for example isophthalic acid or naphthalene dicarboxylic acid. The advantage of this is good thermal stability of the copolyamide. More preferably isophthalic acid is chosen as the dicarboxylic acid. The additional advantage of this is a fairly substantial lowering of the melting point, while mechanical properties are retained.
Suitable diamines are for example linear or branched aliphatic diamines with 2 to 18 carbon atoms. Preferably a linear aliphatic diamine with 7 to 12 carbon atoms is chosen or a branched aliphatic diamine with 6 to 10 carbon atoms. More preferably 1,9-nonane diamine, 1,12-dodecane diamine, trimethylhexamethylene diamine or 2-methylpentamethylene diamine is chosen. The advantage of this choice is that a fairly substantial lowering of the melting point occurs at relatively small amounts of comonomer, while other properties are better retained. Another suitable diamine is for example 1,3-xylylene diamine.
Suitable aminocarboxylic acids or cyclic amides are for example aliphatic aminocarboxylic acids or cyclic amides with 4 to 18 carbon atoms. Preferably 1,11-aminoundecanoic acid, laurolactam or epsilon-caprolactam is chosen. More preferably epsilon-caprolactam is chosen.
A suitable monomer with higher functionality is for example bishexamethylene triamine.
The copolyamide according to the invention can be prepared in various ways known per se for the preparation of polyamides and copolymers thereof. Suitable processes are for example described in Polyamide, Kunststoff Handbuch 3/4, Hanser Verlag (Munchen), 1998, ISBN 3-446-16486-3. Preferably a melt phase process is used in which no (organic) solvents need to be recovered and purified. A disadvantage of reactions in the melt phase may however be the prolonged exposure of the copolyamide to a high temperature. Therefore, use is preferably made of the process in which a mixture of dicarboxylic acids, or esters or polyesters thereof, and diamines, to which mixture water and an excess amount of tetramethylene diamine are added, is polycondensed via the liquid phase to form a low molar mass copolyamide (with a relative viscosity of for example 1.03-1.80). Such a process is for example known from U.S. Pat. No. 5,550,208, EP-0393548-A and EP-0039524-A. This is followed by post-condensation in the solid phase under an inert gas, which may optionally contain steam, until a copolyamide of the desired molar mass and viscosity is obtained. The advantage of such a process is that the copolyamide is in the melt phase at high temperatures for only a short time, so that undesired side-reactions are minimised.
The invention also relates to a plastic composition that contains the copolyamide according to the invention, and optionally contains the usual additives, for example heat- and UV-stabilisers etc., colorants, processing aids, for example mould release agents and lubricants, agents for improving impact resistance, reinforcing fillers and flame retardants. This plastic composition containing the copolyamide may optionally also contain polymers other than polyamides.
The copolyamide or the plastic composition according to the invention is eminently suitable for forming products from the melt, for example by means of injection-moulding, extrusion, blow moulding and compression moulding. Products obtained by using the copolyamide or the plastic composition according to the invention are for example car parts, electric and electronic components, films and fibres.
The invention will be further elucidated with reference to the following examples, without however being limited thereto. A higher relative viscosity (higher than for example 2) can be obtained by choosing other conditions or a different process.